1. Field of the Invention
This invention relates to a process for the purification of alkyl glycosides by distillation, to the mixtures obtained as the products of this distillation process, and to compositions containing the mixtures.
2. Statement of Related Art
Alkyl glycosides are formed by the reaction of monosaccharides with fatty alcohols. The alkyl glycoside products can be used for a variety of purposes, for example, as detergents, wetting agents, gelling agents, lubricants, food emulsifiers, and the like.
The production of lower alkyl monoglycosides by the Fischer process, in which a monosaccharide and a fatty alcohol containing up to 10 carbon atoms in the alkyl group are reacted with one another in the presence of an acidic catalyst, has been known for some time. Such alkyl glycosides are widely used as surfactants. The reaction of the two components with one another is highly selective and the hydrophilicity of the alkyl glycosides obtained is adjustible by varying the molar ratio of fatty alcohol to monosaccharide. Products containing two or more monosaccharide units exhibit higher solubility in water than alkyl monoglycosides.
Processes for the production of higher alkyl monoglycosides and alkyl ollgoglycosides and mixtures thereof, in which the alkyl group contains from 8 to 25 carbon atoms, are also known, for example from U.S. Pat. Nos. 3,722,269; 3,598,865 and 3,839,318. However, the processes described in these publications have several disadvantages so far as industrial syntheses are concerned. The monosaccharides and fatty alcohols employed have only limited miscibility with one another, necessitating the presence of a suitable solvent compatible with both components which has to be separated off in the following step and returned to the process. In addition, oligomers and colored compounds are formed to an often undesirable degree during the reaction and have to be removed at a later stage by elaborate separation procedures. Also, the hydrophilicity of the alkyl glycosides obtained cannot be controlled via the number of monosaccharide units in the molecule.
However, the main disadvantage of state-of-the-art processes is that they can only provide alkyl glycoside/fatty alcohol mixtures in which the ratio of alkyl glycoside to fatty alcohol is from 1:10 to 2:1. Accordingly, the removal of unreacted fatty alcohol from the alkyl glycosides before their practical application is unavoidable, and both solvent extraction and distillation procedures are employed.
According to U.S. Pat. No. 3,839,318, excess fatty alcohol is separated from the glycoside/fatty alcohol reaction mixture by a technically very involved method requiring the use of an additional solvent which must in turn be separated off in an additional process step and, in the interests of economy, must be recovered and returned to the process. Accordingly, this process cannot be carried out in one step. It is therefore preferable to purify crude product alkyl glycosides by distilling off excess fatty alcohol; unfortunately, this is technically difficult because the product alkyl glycosides typically have melting points above 70.degree. C. The melting point of the alkyl glycosides increases with increasing degree of glycosidation, and the viscosity of the products is very high in the vicinity of their melting temperature. Alkyl monoglycosides are only fluid, stirrable, and pumpable above 120.degree. C., while alkyl ollgoglycosides are solids even at higher temperatures. Where thin-layer evaporators are used for the distillation process, as described for example, in U.S. Pat. No. 4,393,203, these viscosity problems are particularly acute: highly viscous contituents of the unpurified reaction mixture solidify in the thin-layer evaporator. Further, most alkyl glycosides show only limited thermal stability. The decomposition reaction begins significantly at temperatures as low as 130.degree. C., resulting in undesirable darkening of the products and in the formation of sugar alcohols. Especially with thin-film reactors, dark-colored degradation products or so-called "sugar charcoal" are soon formed, resulting in a distinct reduction in the quality of the desired products. For these reasons, the process for purifying alkyl glycosides described in U.S. Pat. No. 4,393,203 is limited in its application and can only be applied where the products obtained do not have to meet stringent quality requirements.
U.S. Pat. No. 4,349,669 describes a process for the purification of alkyl glycosides containing from 8 to 16 carbon atoms in the alkyl group by distillation of unreacted fatty alcohols in which the last remnants of fatty alcohol are separated from the product by distillation in the presence of glycols having boiling points higher than those of alcohols to be separated by at most 10.degree. C. and lower by at most 30.degree. C. By addition of the glycol, residues of fatty alcohols are entrained from the product mixture, and also, the product alkyl glycosides are present in dilute and hence low-viscosity form for the duration of the distillation process. The disadvantage of this process from the point of view of industrial manufacturing is that, once again, an entraining agent or solvent, namely a glycol again, an entraining agent or solvent, namely a glycol or a mixture of several glycols, has to be added to the reaction mixture, necessitating an additional step for recovering and recycling this "auxiliary".